1. Field of the Invention
The present invention relates to the field of implantable biocompatible transducers particularly those useful for a fully implantable hearing aid system, and to effecting such transducers' post-implantation operation.
2. Description of the Prior Art
Presently a need exists for implantable, biocompatible transducers for generating an electrical signal in response to a stimulus occurring either within or outside the body. Correspondingly, there also exists a need for effecting a mechanical action within the body in response to an electrical signal. Such biocompatible transducers are useful for cardiac monitoring, drug delivery, or other bodily functions. Biocompatible, implantable transducers that effect a mechanical action with the body may be used in hearing aids, implantable pumps, valves, or for other types of battery energized biological stimulation. Because supplying power for energizing a transducer's operation after implantation is difficult, high-efficiency transducers that require little electrical power are highly desirable. It is also highly desirable that operation of such microactuators be controlled in as simple and as reliable a manner as possible, and that any non-biocompatible components be thoroughly isolated from the body's tissues and fluids without compromising the microactuator's operation.
Particularly for hearing aids, despite a thirty year development effort, it is well recognized that presently available transducers are less than satisfactory hearing aid. A variety of problems such as distortion in the sound generated by the hearing aid itself, discomfort associated with wearing the hearing aid, and social stigma are all significant factors in user dissatisfaction. Even the very best in-the-canal hearing aids, which by themselves may have low distortion in free space, produce appreciable distortion when in use. This distortion, particularly at high sound levels, arises mainly from positive feedback between the hearing aid's microphone and speaker. The present situation is best illustrated by the fact that if an individual with perfectly normal hearing wears a standard hearing aid, speech recognition becomes impossible for a considerable interval until the hearing aid wearer adapts to the prosthesis. An article by Mead C. Killion entitled "The K-Amp Hearing Aid: An Attempt to Present High Fidelity for Persons With Impaired Hearing," American Journal of Audiology, vol. 2, no. 2, July 1993, describes customizing a hearing aid's performance characteristics to meet the unique requirements of each subject's particular hearing loss.
Generally aging produces a hearing loss which cannot be properly compensated by present hearing aids. In most instances, hearing loss occurs generally at higher frequencies. For that reason many hearing aids therefore boost high frequency gain to compensate for this hearing loss. However, such simple techniques inadequately compensate for high frequency hearing loss. The most frequent complaint of hearing aid wearers is the same as that other people who do not wear hearing aids: namely, the inability to discriminate speech in a noisy environment such as at a social gathering, a party, etc. where the hearing aid assistance can be of significant social importance An inability of improve discrimination between noise and a useful signal, typically speech, is a significant problem that severely limits the usefulness of present hearing aids. In such situations, a hearing impaired individual can very clearly hear the acoustic signals, including the desirable ones, but is unable to discriminate or make sense out of them. Conversely, it is well recognized that a person with good hearing can converse with an other person in a noisy environment.
High frequencies present in consonants contain much speech information. With aging, because of high frequency hearing loss, the ability to catch these high frequency cues decreases, and the efficiency of the noise discrimination diminishes. As a result, to capture an intelligible conversation or any signal in a noisy environment such as a party, the hearing impaired individual typically requires that the conversational sound level be approximately 10 to 15 dB above the surrounding noise level. Conversely, it is well known that an individual with good hearing can converse with an other person in a noisy environment, even though the surrounding sound level may be 10 to 15 dB higher than the speech sound level. Although a normal individual may not capture all the sounds in such a noisy environment, even as little as a 45% recognition rate is adequate for filling in the remaining information. The brain therefore provides extremely agile information discrimination in a noisy environment. Unfortunately most present hearing aids equally amplify both conversational sounds and noise. This inability of present hearing aids to improve discrimination distresses most people, and causes about 70% of hearing impaired individuals to eventually either abandon them, or not to purchase one in the first place.
In essence then, beyond faithful reproduction of sound by a hearing aid, it is desirable to discriminate useful sound from the surrounding noise, although it is not always clear that useful sound can be distinguished, a priori, from noise. However, binaural hearing is known to help in discriminating sound. Other methods, such as digital signal processing that apply complex digital filtering techniques selectively to individual frequency bands may improve speech discrimination. However, such digital signal processing is a very complex problem, and its implementation presently requires computationally powerful digital signal processors. However, presently such processors and their associated components cannot be miniaturized sufficiently for use in an implantable hearing aid. Moreover, such digital signal processors consume an amount of electrical power which exceeds that available for a fully implantable hearing aid system that includes an implanted battery designed for a minimum three to five year battery replacement interval.
Patent Cooperation Treaty ("PCT") patent application Ser. No. PCT/US96/15087 filed Sep. 19, 1996, entitled "Implantable Hearing Aid" ("the PCT Patent Application") describes an implantable hearing aid which uses a very small implantable microactuator that employs a stress-biased lead lanthanum zirconia titanate ("PLZT") transducer material. This PCT Patent Application also discloses a Kynar.RTM. microphone which may be physically separated far enough from the implanted microactuator so that no feedback occurs. Embodiments of the microactuator described in this PCT Patent Application disclose how the transducer's deflection or displacement can be magnified, if so desired, by hydraulic amplification. Such microactuators also illustrate how a membrane diaphragm provides good biological isolation for the transducer structure while at the same time fully preserving or actually enhancing transducer performance. This PCT Patent Application also discloses how signals, received by the hearing aid's implantable Kynar microphone, may be used for controlling the hearing aid's operating characteristics. The implantable hearing aid described in the PCT Patent Application, which is extremely compact, sturdy and rugged, provides significant progress towards addressing problems with presently available hearing aids.